Your big problem is that when wheels are cheap, it's because there's fair demand for a wheel with those specs, and the manufacturers are knocking out a ton of them.

MTB has blown up the standards lately. For a while there, everybody was riding about the same thing: 26" rims laced to disc brake hubs with QR axles. So there are a ton of wheelsets that fit that, and they can be pretty cheap.

Long-winded way of saying I don't know a good source for you. Start by figuring out what you need, anyway. 29" rims, obviously, but what are you doing about the freehub? And what axles do you need? Most likely, there are at least a couple sets out there to fit that, though maybe not for $400. At worst, you could be looking at building a set from parts, which gets expensive.